The present invention relates to a switching power source capable of reliably preventing a rush current.
As shown in FIG. 3, a conventional switching power source has a smoothing capacitor C or large capacitance coupled across a rectifier circuit 1 which rectifies an a.c. input from a commercial power source 2. A rush current will flow transiently into the capacitor C when an a.c. input from the commercial power source 2 is supplied upon turning-on of a switch or when an a.c. input temporarily intercepted due to an accident is again supplied. In order to eliminate such a rush current, there is provided a rush current preventing circuit 4 at a power source line 3 of the a.c. input. The rush current preventing circuit 4 is constructed of a parallel circuit of a resistor R1 and a thyristor S1.
An oscillator circuit 5 is constructed of a serial circuit of a primary winding L1 of a power transformer T1 and a collector-emitter path of a transistor Q1. The oscillation state is detected by a winding L2. This oscillation detection signal or trigger signal triggers the thyristor S1 to make it turn on. While the thyristor S1 is turned on, the a.c. input passes through this thyristor S1. The transistor Q1 intermittently cuts the d.c. input smoothed by the capacitor C to thereby generate the oscillation.
The trigger voltage to be applied to the gate of the thyristor S1 is obtained by resistors R2 and R3, and diode D1. Reference numeral 6 represents a control circuit for the oscillator circuit 5.
When an a.c. input is initially supplied upon turning-on of a switch, or when an a.c. input is supplied after a temporary interception which caused the voltage of the capacitor C to drop and the oscillation to stop, the thyristor S1 remains turned off. In such a case, an a.c. input current flows into the rectifier circuit 1 via the resistor R1 so that a large rush current will not flow into the capacitor C.
However, upon recovery from an interception of a commercial power source for a very short period which did not cause the oscillation to stop, a rush current cannot be prevented.
In more particular, a conventional switching power source has been so designed that the oscillation remains continued if an a.c. input interception period is short and the voltage across the capacitor C does not drop from a normal value lower than a predetermined amount. Therefore, if the a.c. input is supplied in this condition, a rush current flow into the smoothing capacitor C through the thyristor S1 which still remains turned on because of the continuation of the oscillation.